1. Field of Invention
This invention relates generally to loudspeaker systems and more particularly to loudspeaker systems including a reflective surface to improve the spatial quality of stereo reproduction.
2. Prior Art
A direct-radiating type of loudspeaker system includes all of the loudspeakers of the system supported at the front of the cabinet of the system and radiating directly towards a preferred listening area. Sound reproduction by a direct-radiating loudspeaker system is commonly perceived as issuing from some point within the cabinet of the loudspeaker system making such reproduction seem artificial and lacking realism. The virtual stage of stereo reproduction including a pair of such loudspeaker systems thus is perceived to be located along a line connecting the pair.
The prior art has been firstly directed to a more realistic type of reproduction by a loudspeaker system compared to that of the direct-radiating type wherein the source of the sound radiated by the loudspeaker system is localized generally in the direction of but at a point in space away from the location of the loudspeaker system.
Typically stereo reproduction occurs in a small room with a volume of about 100 m3. The virtual stage of stereo reproduction in the small room with a pair of direct-radiating loudspeaker systems in front of a listener is thus generally at a much shorter distance from the listener than the distance from an audience to a live performance of music. To more closely approximate the scale of the hearing of a live performance of music, the prior art has been secondly directed to making the apparent distance to the virtual stage of stereo reproduction in a small room to be greater than that which would be provided by means for stereo reproduction including direct-radiating loudspeaker systems.
U.S. Pat. No. 2,710,662 to Camras, 1955 Jun. 14 describes a technique wherein most of the sound of a loudspeaker is firstly directed to the wall of a room that a listener in the room faces or the rear wall. Furthermore, a first reflection of the sound directed to the listener is intended to establish a virtual source of the sound at a location behind the rear wall.
Some research indicates that for the virtual source of sound to be localized at an intended location by a first reflection off of the rear wall of the room, at the location of the listener, the intensity of a first reflection of a sound with respect to that of later arriving reflections must be greater by about 10 dB. In a small room with a volume of about 100 m3, the ratio of distance traveled to the listener of the first reflection of the sound with respect to that of later arriving early reflections tends to be greater than one-third. Such relationship of the ratio of distance traveled will be especially the case where the distance between the loudspeaker and the rear wall is a few meters. Thus a disadvantage of the technique exemplified by U.S. Pat. No. 2,710,662 is that the intensity of the first reflection of the sound relative to that of later arriving reflections may not be sufficiently greater to establish the intended virtual location of the loudspeaker projecting the sound in the room.
Sound projected by a loudspeaker system off of a concave reflective surface to a listener and thus eliminating the listener's perception of the sound emanating from a point source of the sound from within the cabinet of the loudspeaker system is exemplified by U.S. Pat. Nos. 4,190,739 to Torffield, 1980 Feb. 26 and 5,216,209 to Holdaway, 1993 Jun. 1. An apparent additional role of the concave reflective surface according to these patents is focusing to some degree of the sound toward the listener. Neither patent refers to the formation of a virtual sound image, which only under certain conditions accompanies the reflection of sound by a concave reflective surface. As the location of the virtual sound image occurs behind the concave reflective surface, such formation would increase the apparent distance to the source of the sound.
A first embodiment of U.S. Pat. No. 4,190,739 to Torffield includes a partly or entirely roughened concave reflective surface. The roughening is intended to partially diffuse high frequency sound reflected by the reflective surface of this first embodiment. Such partial diffusing largely negates the formation of a virtual sound image of the high frequency sound. A second embodiment of this patent includes placement of the center of radiation of a loudspeaker coincident with the focal point of the concave reflective surface that the loudspeaker system projects sound toward. Such placement negates the formation of a virtual sound image, as the propagation of the sound reflected off of the reflective surface is then parallel to the principal axis of the reflective surface.
U.S. Pat. No. 5,216,209 to Holdaway teaches that a loudspeaker should be positioned at a distance from the vertex of a concave reflective surface that the loudspeaker projects sound toward causing the sound rays emanating from the reflective surface to diverge from the principal axis of the concavity of the surface. Given the occurrence of such divergence, then a virtual sound image is formed. The location of the virtual sound image should be at a distance behind the reflective surface equal to a few meters for the distance to the image to be perceptible to a listener. This patent does not teach the relationship between the curvature of the concavity of the reflective surface and the distance of the center of radiation of the loudspeaker to the vertex of that curvature affecting the distance of the virtual image behind the reflective surface.
U.S. Pat. No. 4,190,739 to Torffield states that the reflective surface of a reflector according to this patent can possibly be a concavity both horizontally and vertically. However, this patent does not teach the conditions under which a concavity horizontally or vertically might be eliminated nor does this patent refer to an embodiment not including a reflective surface with a concavity both vertically and horizontally. U.S. Pat. No. 5,216,209 to Holdaway stipulates that the reflective surface according to this patent includes a concavity both horizontally and vertically. The reflective surface of a reflector including a concavity both horizontally and vertically presents greater difficulties and thus expense in the manufacturing of the reflective surface than a reflective surface that can be flat in one direction.
U.S. Pat. No. 4,190,739 to Torffield proposes a reflector with a reflective surface area between 5 to 8 feet square for use in a listening room in a home. The reflector according to this patent can be positioned orthogonal with respect to the direction of and to one side of a listener facing forward. For a listening room with a volume of about 100 m3 for example in a home, to save space, his reflector would be best attached to a wall of the listening room. His reflector with a concavity both horizontally and vertically, however, in all likelihood would be of a weight requiring extraordinary means for securely attaching it to the wall.
According to U.S. Pat. No. 5,216,209 to Holdaway, for a room of standard size in a home, the reflector screen of this patent can measure about 48 inches in width. The curvature of Holdaway's reflector screen horizontally is symmetrical with respect to a central axis and loudspeakers radiating toward the screen are positioned close to the central axis. The propagation of sound reflected from his screen, then, is generally in the direction of the central axis of the concavity of the screen. Thus, for the purpose of reflecting sound toward a listener generally near the middle of the room, his screen must be horizontally positioned obliquely in the room. The size and oblique positioning of his reflector screen may result in his screen occupying such a substantial percentage of the floor area of the room as to be impractical or unappealing.